In response to people's topic posts, I didn't pick Ditto or Azurill for one specific reason, they're have other duplicates of what makes them special. For example, Transform is exclusive to Ditto? WRONG! Mew can learn Transform also, and that's where the whole 'Ditto are failed clones of Mew' conspiracy came from. Oh, and it's still in the same Evolutionary line, but Marill can change gender upon evolution as well.